It depends who you ask. The OSI have a definition (http://www.opensource.org/osd.html http://www.opensource.org/osd.html). It isn't the only definition, but it'

It depends who you ask. The OSI have a definition (http://www.opensource.org/osd.html http://www.opensource.org/osd.html). It isn't the only definition, but it's probably the only definition self-important enough to have its own webpage. Some people get angry if you don't agree with that definition, but there's no real reason to consider it privileged above the lay definition ("the source is available").